Nature of Deposit and Mode of Their Formation

Gasoline and diesel fuels contain highly branched olefins and aromatics which burns to form peroxides, hydroperoxides, and free radicals. The fuel degradation products go past the piston rings into the lubricant as blow-by and attack largely the hydrocarbon lubricant. The reaction between the contents of the blow-by and the lubricant olefins and aromatics results in the formation of the lubricant-derived peroxides and hydroperoxides that either oxidatively or thermally decompose to form aldehydes, ketones, and carboxylic acids.

The reaction between nitrogen and oxygen to form NO_x is more prevalent in diesel engines and gasoline engines that are subjected to severe service, such as long-distance driving for extended periods. The NO_x formation initiates when the temperature reaches 137°C. Acids can also result from oxidation of the fuel sulfur.

The aldehydes and ketones undergo aldol-type condensation in the presence of bases or acids to form oligomeric or polymeric compounds. These can further oxidize to highly oxygenated hydrocarbons, commonly referred to as oxygenates. The oxygenates are usually of sticky consistency, and the term resin is often used to describe them. Resin is either the basic component in or the precursor to all types of deposits. Common types of deposits include varnish, lacquer, carbon, and sludge. Varnish, lacquer, and carbon occur when resin separates on hot surfaces and dehydrates or polymerizes to make tenacious films. The quantity and the nature of deposits depend on the proximity of the engine parts to the combustion chamber.

Carbon Deposits:

The parts closer to the combustion chamber, such as exhaust valve head and stem that experience approximate temperatures of 630–730°C, will develop carbon deposits. The same is true of the combustion chamber wall, piston crown, top land, and top groove, which are exposed to approximate temperatures of 200–300°C. Carbon deposits are more common in diesel engines than in gasoline engines and result from the burning of the liquid lubricating oil and the high-boiling fractions of the fuel that adhere to hot surfaces.

Lacquer and Varnish:

As we move away from these regions to the low-temperature regions, such as the piston skirt, the deposits are not heavy and form only a thin film. For diesel engine pistons, this type of deposit is referred to as lacquer; for gasoline engine pistons, this type of deposit is called varnish. The difference between lacquer and varnish is that lacquer is lubricant-derived and varnish is largely fuel-derived. In addition, the two differ in their solubility characteristics. That is, lacquer is water soluble and varnish is acetone-soluble. Lacquer usually occurs on piston skirts, on cylinder walls, and in the combustion chamber, whereas varnish occurs on valve lifters, piston rings, piston skirts, valve covers, and positive crankcase ventilation (PCV) valves.

Sludge:

The coolest parts of the engine, such as rocker arm covers, oil screen, and oil pan, that are exposed to temperatures of ≤200°C experience sludge deposits. Sludge can be watery or hard in consistency, depending on the severity of service. If the service is extremely mild and of short duration, as in the case of stop-and-go gasoline engine operation, the sludge is likely to be watery or mayonnaise like. This type of sludge is called low-temperature sludge, which occurs when the ambient temperature is <95°C. The high-temperature sludge is more common in diesel engines and gasoline engines with long, continuous operation. This type of sludge occurs when the ambient temperature is >120°C and is hard in consistency. In the former case, the engine does not get hot enough to expel combustion water, which stays mixed with oil, imparting sludge, a mayonnaise like appearance. In the latter case, however, the ambient temperature is high enough to expel water, thereby resulting in hard sludge. Sludge is common in areas that experience low oil flow, such as crankcase bottoms and rocker boxes.

Soot:

Another component of the combustion effluent that must be considered is soot. Soot not only contributes toward some types of deposits such as carbon and sludge, but it also leads to a viscosity increase. These factors can cause poor lubricant circulation and lubricating film formation, both of which will result in wear and catastrophic failure. Soot is particulate in nature and results from the incomplete combustion of the fuel and of the lubricating oil from the crankcase that might enter the combustion chamber by traveling past the piston rings. Fuel-derived soot is a chronic problem in the case of diesel engines because diesel fuel contains high-boiling components that do not burn
easily. In addition, diesel engine combustion is largely heterogeneous, with poor air–fuel mixing, hence poor combustion. Soot is made of hydrocarbon fragments with some of the hydrogen atoms removed. The particles are charged and hence have the tendency to form aggregates. When aggregates occur on surfaces, such as those of the combustion chamber, soot deposits result. These deposits are soft and flaky in texture. If these occur in oil, lubricant experiences an increase in viscosity. A soot-related viscosity increase usually requires the presence of polar materials in oil that have the ability to associate with soot. These can be additives or polar lubricant oxidation and degradation products. Carbon deposits are lower in carbon content than soot and, in most cases, contain oily material and ash. This makes knowledge of the ash-forming tendency of a lubricant important to a formulator. When soot associates with resin, one gets either resin-coated soot particles or soot-coated resin particles. The first type of particles results when resin is in excess, and the second type results when soot is in excess. The amount of soot in resin determines the color of the deposits: the higher the soot, the darker the deposits. Sludge results when resin, soot, oil, and water mix.